IT IS more than 10 years since Carl Sagan and Ann Druyan's best-selling book-of-the-television-series, Cosmos, was published, which is a long time to come up with a sequel. What has happened in the interim, of course, is that popular science writers and their publishers have been newly inspired by the quest to see if there are any more crocks of gold where Stephen Hawking's came from.

Sagan and Druyan have turned from the great galactic outback to the microcosmic business of human evolution. They record that 44 per cent of their fellow Americans, when polled, claim to believe that we were created, just as we are, by an invisible extra-terrestrial being on a sunny Saturday a mere 5,000 years ago. To put the other case, the authors broadly devote the first half of the book to DNA and Charles Darwin and the second half to chimpanzees.

They almost succeed in making DNA comprehensible, although there must be a better way to explain how this monster molecule causes the formation of other molecules around it than by way of a woolly metaphor about 'job orders and blueprints for molecular machine tools.'

The potted life of Darwin puts a useful weight on his discovery that any fool can see the evolutionary process in action down on the farm. We have produced new and improved cattle, horses, dogs and crops by selective breeding in a few hundred years. Give the slower, unassisted method of natural selection a few million years and anything can happen. A single-cell protozoan's lineal heir and descendant can turn out to be Wilberforce or Huxley. We also learn that Darwin's academic grounding was a monumental eight-year study of barnacles. Waiting for one of them to move, possibly.

Sagan and Druyan present the great Huxley-Wilberforce debate at Oxford as a scene in a Thirties Hollywood film script, imagining stars such as Ronald Colman and Spencer Tracy in the various parts. This is a bad, patronising move. But their attempt at a chimp's-eye-view ('The Big Guy, he gets respect. He walks by, folks bow') works well.

The book's worst drawback is its faddish ascription of all the world's ills to testosterone. 'Intact males,' Sagan and wife assure us, 'are an awkward necessity.' They detail the rape-and-pillage lifestyle of male chimpanzees at length and contrast it with female decency, even where this means falsifying the record. Female chimps cheerlead at intertribal fights, go into an erotic frenzy at the shedding of blood and give instant sexual rewards to the victors. You'd never know from reading this book.

You'd never know, either, that the key hormone in human aggression is not testosterone, of which men produce 10 times more than women, but noradrenaline, of which the sexes produce equal amounts.

Testosterone may be more important in chimps, but then they have far higher levels of it than we do, so Sagan and Druyan are using a cracked mirror when they hold up the chimpanzee world as a reflection of our own origins. They do point out that we have had no common ancestry with chimps since about eight million years ago, and that no given chimpanzee trait is necessarily in our blood as well; they then immediately draw on their observations about chimp sexuality to claim that all men are potential rapists, which is profoundly irritating, despite their concession that, by the same token, women exhibit 'the compliance pattern of female chimps' and encourage dominant behaviour in men. All this gets us nowhere.

The authors regard the prohibitions that human beings place on things such as rape as 'a kind of stencil' laid over our 'inherited predispositions'. But our inherited predisposition to co-operate with others and obey the social rules is surely our strongest of all. It can cause massacres, but it is also the one thing that can stop them.

There is plenty of illumination and a certain amount of wisdom in Shadows of Forgotten Ancestors, so it's a shame that Sagan and Druyan make such intellectual fashion-victims of themselves with their hormone argument. At the very end they give some overdue consideration to the social factor: 'Our ancestors have bequeathed us - within certain limits, to be sure - the ability to change our institutions and ourselves.' They recommend this procedure, on the grounds that Nature has no benevolent grand plan and it is getting late if we want to avoid extinction. 'But by what right, what argument, can pessimism be justified? . . . Nothing is preordained.' Not even by our knuckle- dragging bad-boy genes, apparently.